Cognate with Old Japanese verb 築く (tsuku, “to build up using earth and/or stone”).[1]

The /a/ ending may indicate that tsuka developed as the nominalization of the 未然形 (mizenkei, “irrealis form”) of the verb, suggesting an original meaning of "that which is being built up into an earthworks (but isn't finished yet)". The irrealis is also the root form for constructing the passive form of all Japanese verbs, so the original meaning might instead have been just the passive sense of "that which is built up into an earthworks".